NASA's Hubble Space Telescope has measured the largest object in the
solar system ever seen since the discovery
of Pluto 72 years ago.

Approximately half the size of Pluto, the icy world 2002 LM60, dubbed
"Quaoar" (pronounced kwa-whar) by its discoverers, is the farthest
object in the solar system ever to be resolved by a telescope. It
was initially detected by a ground-based telescope, as simply a dot
of light, until astronomers aimed the powerful Hubble telescope at it.

Quaoar is about 4 billion miles away from Earth, well over a billion
miles farther away than Pluto. Unlike Pluto, its orbit around the
Sun is very circular, even more so than most
of the planetary-class bodies in the solar system.

Although smaller than Pluto, Quaoar is greater in volume than all
the asteroids combined (though probably
only one-third the mass of the asteroid belt, because it's icy rather
than rocky). Quaoar's composition is theorized to be largely ices
mixed with rock, not unlike that of a comet, though 100 million
times greater in volume.

This finding yields important new insights into the origin and
dynamics of the planets, and the mysterious population of bodies
dwelling in the solar system's final frontier: the elusive, icy
Kuiper belt beyond Neptune.

Michael Brown and Chadwick Trujillo of Caltech are reporting the
findings today at the 34th annual meeting of the Division for
Planetary Sciences of the American Astronomical Society in Birmingham, Ala.

Earlier this year, Trujillo and Brown used the Palomar Oschin Schmidt
telescope to discover Quaoar as an 18.5-magnitude object creeping
across the summer constellation Ophiuchus (it's less than 1/10,000th
the brightness of the faintest star seen by the human eye). Brown had
to do follow-up observations using Hubble's new Advanced Camera for
Surveys on July 5 and August 1, 2002, to measure the object's true
angular size of 40 milliarcseconds, corresponding to a diameter of
about 800 miles (1300 kilometers). Only Hubble has the sharpness
needed to actually resolve the disk of the distant world, leading
to the first-ever direct measurement of the true size of a Kuiper
belt object (KBO).

Like Pluto, Quaoar dwells in the Kuiper belt, an icy debris field
of comet-like bodies extending 7 billion miles beyond Neptune's orbit.
Over the past decade more than 500 icy worlds have been found in the
Kuiper belt. With a few exceptions all have been significantly smaller
than Pluto.

Previous record holders are a KBO called Varuna, and an object called
2002 AW197, each approximately 540 miles across (900 kilometers). Unlike
Hubble's direct observations, these diameters are deduced from measuring
the objects' temperatures and calculating a size based on assumptions
about the KBOs' reflectivity, so the uncertainty in true size is much greater.

This latest large KBO is too new to have been officially named by the
International Astronomical Union. Trujillo and Brown have proposed naming
it after a creation god of the Tongva native American tribe, the original
inhabitants of the Los Angeles basin. According to legend, Quaoar, "came
down from heaven; and, after reducing chaos to order, laid out the world
on the back of seven giants. He then created the lower animals, and then
mankind."

Quaoar's "icy dwarf" cousin, Pluto, was discovered in 1930 in the course
of a 15-year search for trans-Neptunian planets. It wasn't realized until
much later that Pluto actually was the largest of the known Kuiper belt
objects. The Kuiper belt wasn't theorized until 1950, after comet orbits
provided telltale evidence of a vast nesting ground for comets just beyond
Neptune. The first recognized Kuiper belt objects were not discovered
until the early 1990s. This new object is by far the "biggest fish"
astronomers have snagged in KBO surveys. Brown predicts that within
a few years even larger KBOs will be found, and Hubble will be invaluable
for follow-up observations to pin down sizes.